1. Field Of The Invention
This invention relates to a process for producing bent, double wall rigid pipes.
2. Description Of The Prior Art
If an exothermal or endothermal reaction takes place inside the inner pipe and if cooling or heating is required, such a heat exchanger is known as a tubular reactor.
The hollow space between the inner and outer pipes of the double-walled tubular reactor serves for the passage of coolants or heating agents for the purpose of influencing the reaction temperature inside the tubular reactor. In order to achieve as uniform an effect of the coolant or heating agent as possible, attempts are made to achieve concentricity of inner and outer pipes, since this would create defined flow conditions in the hollow space between the pipes.
Whereas it is easy to meet this requirement in the case of straight pipes, considerable difficulties are encountered in the case of elbows which are normally 180-degree bends. The elbow-shaped cold-bent high-pressure pipes for operating pressures of over 1000 bar have a bending radius of about 7 times the high-pressure pipe diameter and larger, and have a straight piece of pipe of at least 5 times the outside diameter on each shank. This straight piece of pipe is required for the screwed joint with the next straight piece of pipe and for reasons of fabricating technique. It has further been found that the bending radius cannot be kept constant over the 180-degree bend, i.e. it fluctuates within a tolerance of .+-.0.10 m to 0.20 m. The reason for such non-uniformity is due to the fact that the high-pressure pipes are cold-bent and that they have different elastic recovery. Subsequent adjustment to the exact bending radius is not permissible.
The process used hitherto for jacketing a high-pressure pipe of the type described above consisted of bending the outer pipe by known means separately at a bending radius equal to the specified bending radius of the high-pressure pipe. The resulting outer pipe bend, which did not have any extended straight shanks, was then cut through either at the two lateral neutral bending lines or at the inner and outer phase of the bend. The two half-shells thus obtained were then placed around the high-pressure bend and adjusted to the actual shape of the bend by applying heat and by pressing. The cuts were then welded. This was followed by welding straight pieces of pipe to the extremities of the outer pipe shanks. The high-pressure inner pipe was centered in the center pipe by means of spacers.
The process used hitherto for jacketing an inner pipe is too costly and time-consuming because of the necessity to cut the outer bend and subsequently to weld the two half-shells together again. Moreover, in the course of welding, the outer pipe becomes distorted, resulting in a non-uniform hollow space within the bend. This will, in turn, result in non-uniform flow velocities of the coolant or heating agent, which fact will affect the heat transfer, thus causing unequal temperatures at various points within the high-pressure pipe bend. Besides the stresses caused by the high operating pressure, there will also be additional stresses due to temperature differences, which will have an adverse effect.